Integrated heat spreader and method of fabrication

ABSTRACT

An integrated heat spreader is disclosed in which grooves are formed in a recess of the heat spreader to enhance the stiffness and strength of the integrated heat spreader without increasing production costs or complexity. The integrated heat spreader may be fabricated by providing a metal strip having raised portions thereon to provide a recess therebetween, forming grooves on a bottom surface of the recess, where the grooves extend along a periphery of the bottom surface which is substantially free of the raised portions, and subsequently singulating an integrated heat spreader from the metal strip.

BACKGROUND

1. Technical Field

Embodiments of the invention relate to heat dissipation devices forelectronic packages, and more particularly to an integrated heatspreader. Disclosed embodiments also include a method of fabricating theintegrated heat spreader structure using commonly available pressequipment.

2. Description of Related Art

Currently, integrated heat spreaders are fabricated from copper stripwhich has single plane thicknesses. The integrated heat spreaders aretypically fabricated from raw copper strip by cold forging processes toform raised rims around the integrated heat spreaders. Such processesinvolve heavy press equipment, e.g. a 250 to 400 ton press, which isless commonly available. Further, heavy press equipment requiressubstantial capital investment, thereby constraining flexibility involume expansion. Lighter press equipment may not be appropriateespecially if a large size heat spreader, e.g. 50 mm×50 mm is desired.

Integrated heat spreaders fabricated from raw copper strip requireplating with nickel or other materials to prevent corrosion. However,where the integrated heat spreaders are fabricated using cold forging,nickel plating can only be performed on the completed heat spreaders asthe press equipment used in cold forging may damage any pre-platednickel due to excessive pressure exerted by the press equipment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow sequence of fabricating an integrated heat spreaderaccording to one embodiment of the invention.

FIG. 2A illustrates a stamping process for forming a dual gauge metalstrip from a single gauge metal strip.

FIG. 2B is a side view of a metal strip prior to the stamping process ofFIG. 2A.

FIG. 2C is a side view of the metal strip after the stamping process ofFIG. 2A.

FIG. 2D is a top view of the metal strip at the stamping table.

FIG. 3 illustrates stamping the metal strip by a light ton press.

FIG. 4 illustrates forming grooves in the metal strip according to oneembodiment of the invention.

FIG. 5 illustrates singulating an integrated heat spreader from themetal strip according to one embodiment of the invention.

FIG. 6A is a perspective view of an integrated heat spreader having alid-shape cross-sectional profile.

FIG. 6B is a side view of a package incorporating the integrated heatspreader of FIG. 6A.

FIG. 7A is a perspective view of an integrated heat spreader having ahat-shape cross-sectional profile.

FIG. 7B is a side view of a package incorporating the integrated heatspreader of FIG. 7A.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

In the following description, numerous specific details are set forth inorder to provide a thorough understanding of various illustrativeembodiments of the present invention. It will be understood, however, toone skilled in the art, that embodiments of the present invention may bepracticed without some or all of these specific details. In otherinstances, well known process operations have not been described indetail in order not to unnecessarily obscure pertinent aspects ofembodiments being described. In the drawings, like reference numeralsrefer to same or similar functionalities or features throughout theseveral views.

FIG. 1 is a flow chart summarizing a sequence 100 of fabricating anintegrated heat spreader according to one embodiment of the invention.The process sequence 100 will be described with further reference toFIGS. 3 to 5 illustrating various process outputs obtained during theprocess sequence 100 of FIG. 1.

The process sequence 100 begins with providing a metal strip 20 b withraised portions 22 (block 102). The metal strip 20 b may be dual ormultiple gauge, in which the strip 20 b has two or more cross-sectionalthicknesses or raised portions interposing a recess therebetween. Themetal strip 20 b may comprise copper, aluminum, or other suitable metalsor metal alloys.

In order to provide the dual gauge metal strip 20 b, a metal strip 20 ahaving a single thickness, may be subject to a stamping or a rollingprocess to form multiple raised portions 22. FIG. 2 illustrates astamping process in which a single gauge (single thickness) metal strip20 a is processed into a dual gauge (double thickness) metal strip 20 bhaving a lid-shape profile. The metal strip 20 a may be passed through astamping table 24 where multiple (e.g. three) stamping dies 26 orrollers are operable to form two raised portions 22 from the metal strip20 a. FIG. 2A illustrates a stamping process involving a single stampingstep for forming a dual gauge metal strip 20 b from a single gauge metalstrip 20 a. FIGS. 2B and 2C are side views of the metal strip prior toand after the stamping process. FIG. 2D is a top view of the metal strip20 a at the stamping table 24.

If required, a dual gauge metal strip 20 b having a hat-shape profilemay be fabricated using two or more stamping steps. A first stampingstep may be required to form two raised portions 22 interposing a recessas illustrated in FIG. 2C, and a second stamping step may be required toform a brim of the hat shape.

After providing a metal strip 20 b with dual or multiple raised portions22, the process sequence 100 may then proceed to preparing the metalstrip 20 b to meet specific physical requirements (block 104), includingbut not limited to flatness and dimensions. To this purpose, the metalstrip 20 b may be stamped or rolled by a light ton press 28, e.g. 80 tonto 125 ton, to substantially even surfaces of the raised portions or therecess therebetween or both (see FIG. 3). Dimensional requirements ofthe integrated heat spreader may also be adjusted through stamping.

The process sequence 100 may then proceed to forming a plurality ofgrooves 30 on a bottom surface 32 of the recess (block 106). The bottomsurface 32 of the recess has a periphery, wherein a portion issurrounded by the raised portions 22 while the remaining portion of theperiphery is substantially free of the raised portions 22. Grooves 30may be formed along the portion of the periphery of the bottom surface32 which is substantially free of the raised portions 22. The grooves 30may extend partially between the raised portions 22. In one embodiment,the length of the grooves (L_(G)) may be between about 70% to about 90%of the recess length (L_(R)) which refers to the direct distance betweenthe raised portions 22. To this purpose, cutting knives 34 or cuttingtools may be arranged at appropriate positions and operable to formgrooves 30 on the bottom surface 32 of the recess (see FIG. 4). Thegrooves 30 may partially penetrate the bottom surface 32 of the recessat a depth between one tenth to one quarter of a height or thickness(T_(R)) of the metal strip 20 b at the recess. In the embodiments of theappended drawings, the grooves 30 are illustrated as single slitsarranged on opposite sides of the periphery of the bottom surface 32 inthe recess. In other embodiments, the grooves 30 may be in the form ofnon-contiguous slits or perforations.

After the grooves 30 are formed, the process sequence 100 may thenproceed with singulating an integrated heat spreader 40 from the metalstrip 20 b (block 108). To this, the metal strip 20 b may be severed atthe raised portions 22 according to dimensional requirements to separatean integrated heat spreader 40 from the metal strip 20 b (see FIG. 5).The integrated heat spreader 40 singulated from the metal strip 20 b maybe rendered for further processing or for use as required.

After singulation, the integrated heat spreader 40 may be plated with acorrosion resistant material, e.g. nickel, using methods such aselectrical or electroless plating to protect the heat spreader fromadverse environmental effects. Alternatively, the corrosion resistantmaterial may be pre-plated on the metal strip 20 b prior to fabricatingthe integrated heat spreader 40. Pre-plating offers cost advantages asthe plating is performed on the metal strip 20 b from which multipleintegrated heat spreaders 40 may be fabricated. In embodiments where themetal strip 20 b is pre-plated with a corrosion material, the completedintegrated heat spreader 40 would include side edges, along thesingulation sites, which are substantially free of the plated material.

If required, the integrated heat spreader 40 may also be spot platedwith a material having high electrical conductivity, e.g. gold, toimprove the wettability of a thermal interface material 62 interposedbetween the integrated heat spreader 40 and a semiconductor die 60.

Stop

Reference is made to FIG. 6A illustrating a perspective view of anintegrated heat spreader 40 having a lid-shape cross-sectional profile.Reference is also made to FIG. 7A illustrating a perspective view of anintegrated heat spreader 50 having a hat shape cross-sectional profile.The integrated heat spreader 40, 50 has a center region surrounded by anouter region which includes at least raised portions 22 interposing thecenter region to provide a recess therebetween. The bottom surface 32 ofthe recess includes a plurality of grooves 30 extending along a portionof its periphery which is substantially free of the raised portions 22.The grooves 30 may be arranged proximate to the edge of the integratedheat spreader 40, 50 such that when the integrated heat spreader 40, 50is mounted on a semiconductor die 60, the grooves 30 do not overlay thesemiconductor die 60. Accordingly, dimensions of the center region ofthe integrated heat spreader 40, 50 may be larger than a top surface ofthe semiconductor die 60.

Reference is made to FIGS. 6B and 7B illustrating side views ofsemiconductor packages 70, 80 incorporating the integrated heatspreaders 40, 50 of FIGS. 6A and 7A respectively. The integrated heatspreader 40, 50 may be mounted on a semiconductor die 60 using a thermalinterface material 62 interposed therebetween to increase thermaltransfer efficiency. The semiconductor die 60 is mounted on a packagesubstrate 64 using interconnects 66, e.g. solder balls, and may beprovided with an underfill material 68 to protect the interconnects 66from the ambient environment. If power dissipation requirements arehigh, an external heat sink (not shown) may be mounted on the integratedheat spreader 40, 50.

Embodiments of the invention are advantageous in reinforcing themechanical strength of the package substrate. When organic packages aresubjected to temperature changes, the substrates are prone to (concaveor convex) warpage due to mismatch in coefficients of thermal expansion(CTE) of the constituent materials. The integrated heat spreaderprevents substrate warpage by providing mechanical reinforcement to thesubstrate. Further, grooves formed in the recess of the integrated heatspreader are advantageous in improving the stiffness of the integratedheat spreader, thereby also improving the mechanical strength of thepackage.

Further advantages of embodiments of the invention include lowermanufacturing costs due to the following factors. Lighter pressequipment, e.g., 80 to 125 ton press which is commonly available in theindustry may be used even for large-size integrated heat spreader, e.g.50 mm×50 mm without compromising the stiffness and strength of theintegrated heat spreader. Further, the multiple gauge metal strip may bepre-plated prior to fabricating the integrated heat spreader to increaseproduction capacity and yet decrease plating costs.

Other embodiments will be apparent to those skilled in the art fromconsideration of the specification and practice of the presentinvention. Furthermore, certain terminology has been used for thepurposes of descriptive clarity, and not to limit the invention. Theembodiments and features described above should be considered exemplary,with the invention being defined by the appended claims.

1. A method comprising: providing a metal strip having at least tworaised portions thereon to provide a recess therebetween, forming aplurality of grooves on a bottom surface of the recess, the groovesextending along a periphery of the bottom surface which is substantiallyfree of the raised portions; and singulating an integrated heat spreaderfrom the metal strip.
 2. The method of claim 1, wherein singulating theintegrated heat spreader is by cutting the metal strip at the raisedportions.
 3. The method of claim 2, further comprising plating the metalstrip with a corrosion resistant material prior to forming the grooves.4. The method of claim 3, wherein each of the plurality of grooves hasdepth between one tenth to one quarter of a thickness of the metal stripat the recess.
 5. The method of claim 4, wherein the heat spreader isplated with a corrosion resistant material prior to forming the grooves.6. The method of claim 4, wherein a side surface of each of the raisedportions is substantially free of the corrosion resistant material. 7.The method of claim 6, wherein a cross-sectional profile of the heatspreader is one of a lid shape and a hat shape.
 8. The method of claim6, wherein the grooves formed on the bottom are non-contiguous. 9.-14.(canceled)
 15. A method comprising: providing a metal strip having atleast two raised portions thereon to provide a recess therebetween,forming a plurality of grooves on a bottom surface of the recess, thegrooves extending along a periphery of the bottom surface which issubstantially free of the raised portions, wherein the grooves have alength between about 70% to 90% of a length between the at least tworaised portions; and singulating an integrated heat spreader from themetal strip.
 16. The method of claim 15, wherein singulating theintegrated heat spreader is by cutting the metal strip at the raisedportions.
 17. The method of claim 16, further comprising plating themetal strip with a corrosion resistant material prior to forming thegrooves.
 18. The method of claim 17, wherein each of the plurality ofgrooves has depth between one tenth to one quarter of a thickness of themetal strip at the recess.
 19. The method of claim 18, wherein the heatspreader is plated with a corrosion resistant material prior to formingthe grooves.
 20. The method of claim 18, wherein a side surface of eachof the raised portions is substantially free of the corrosion resistantmaterial.
 21. The method of claim 20, wherein a cross-sectional profileof the heat spreader is one of a lid shape and a hat shape.
 22. Themethod of claim 20, wherein the grooves formed on the bottom arenon-contiguous.